480 Anatomy of the posterior approach to the lumbar plexus Juliana Farny MD,* Pierre Dmlet mD FRCPC,~ block Michel Girard MD MHPE FRCPC~ The purpose of this study was to describe the relation of the qu'entre celui-ci et le carrd des lombes. Le nerf fdmoral est lumbar plexus with the psoas major and with the superficial situd entre les nerfs fdmoro-cutand latdral et obturateur. Ce- and deep landmarks close to it. Four cadavers were dissected pendant, alors que le nerf fdmoro-cutand latdral est dans le and 22 computed tomography files of the lumbosacral region m~me plan que le nerf fkmoral, le nerf obturateur peut les studied. Cadaver dissections demonstrated that the lumbar y accompagner ou se retrouver dans un repli du muscle psoas plexus, at the level of L 5, is within the substance of the psoas quile sdpare des deux autres nerfs. Ins dtudes radiologiques major muscle rather than between this muscle and the quad- ont fourni les donndes suivantes: le neorfdmoral est it une pro- ratus lumborum. The femoral nerve lies between the lateral fondeur de 9,01 + 2,43 cm; le bord interne du psoas est it femoral cutaneous and obturator nerves. However, while the 2, 73 + 0,64 cm du plan sagital mddian; alors que le bord externe lateral femoral cutaneous nerve is in the same fascial plane de ce muscle est it 6,41 + 1,61 cm de ce m~me plan. Nous as the femoral nerve, the obturator nerve can be found in the concluons que le plexus lombaire est clans le muscle psoas, same plane as the two other nerves or in its own muscular que la position du neff obturateur varie par rapport au neff fold. Radiological data provided the following measurements: fdmoral et que la tomographie aide it situer le plexus par rapport the femoral nerve is at a depth of 9.01 + Z43 cm; the psoas it d'autres repbres. major medial border is at 2.73 + 0.64 cm from the median sagital plane; and its lateral border is at 6.41 + 1.61 cm from the same plane. It is concluded that the lumbar plexus is within A knowledge of the anatomy is essential to an anaesthetist the psoas major, that the obturator nerve localization within who wishes to perform a regional anaesthesia techique the psoas major varies and that computed tomography data and the posterior approach to the lumbar plexus is no define precisely the relationship of the lumbar plexus with su- exception. The literature on lumbar plexus anatomy con- perficial and deep landmarks. tains contradictory information. Winnie' and Wedel 2 lo- cate the lumbar plexus between the quadratus lumborum Le but de ce travail dtait de ddcrire la relation anatomique and psoas major muscles. However, a number of anat- entre le plexus lombaire et le muscle psoas, de m~me qu'avec omy textbooks, 3-6 Chayen 7 and more recently a radio- des repbres anatomiques superficiels et profonds. Quatre ca- logical study by Dietemann and Sick s place the lumbar davres ont dtd dissdquds et 22 tomographies de la rdgion lom- plexus within the substance of the psoas major. bosacrde dtudi$es. Les dissections montrent que le plexus lom- The goal of the study was to describe precisely the baire, au niveau de Lj, se trouve dans le muscle psoas plut6t anatomical course of the lumbar plexus and its relation- ship with adjacent structures and with surface and deep anatomical landmarks. Key words Methods ANATOMY; lumbar plexus; The protocol was reviewed and approved by the research ANAESTHETICTECHNIQUES" regional, lumbar plexus and ethics committee of our institution. This work con- block, sciatic nerve block. sists of two parts. In the f~t, four adult fresh cadavers From the Dtpartement d'anesthtsie-rtanimation, tHtpital (two women and two men) were dissected to describe Maisonneuve-Rosemont et Universit6 de Montrtal, Montrtal, the topography of the lumbar plexus and its branches. Qutbec and *CHRU de Strasbourg, France. After the abdominal cavity was emptied of viscera, a me- Address correspondence to: Dr Pierre Drolet, Dtpartement ticulous dissection of the two psoas major muscles be- d'anesthtsie-rtanimation, H6pital Maisonneuve-Rosemont, tween l-a-L3 and SoS2 was carded. The lumbar plexus 5415 boul. L'Assomption, Montrtal, Qutbec, HIT 2M4. and its branches relationships with other landmarks were Accepted for publication 3rd March, 1994. noted. Once the dissection was finished, the muscles were CAN J ANAESTH 1994 / 41:6 / pp480-5 Farny el al.: LUMBAR PLEXUS ANATOMY 481 FIGURE 1 Dissectiondata collectionlandmarks. (1) Distance skin/transverseprocess Ls, (2) distanceskin/lumbar plexus nervoustrunks, (3) distance nervoustrunks/median sagital plane, (4) distance psoas major lateral border/median sagital plane, (5) psoas major frontaldiameter, (6) psoas major median sagital diameter. placed in their original position and a transverse section of the psoas major. The plexus displayed a triangular was made at the level of the L4-5 intervertebral disc. Meas- shape, narrow in its superior portion and wider in its urements (Figure I) were made from the transverse section. lower portion. It passed anteriorly to the lumbar trans- In the second part, axial transverse sections of com- verse processes within the substance of the psoas major. puted tomography of the lumbosacral region (Ll to $2-3) It lay between: (a) the fleshy slips of the main part of were used to compute the following measurements and the psoas major that arose from the antero-lateral part distances: subcutaneous fat thickness, intervertebral disc of the vertebral bodies and the intervertebral disc and diameter, distance of the skin to the transverse process, (b) the accessory part of the psoas, posterior to the main distance of the medial aspect of the psoas to the median part of the muscle, that originated from the anterior part sagital plane, distance of the lateral aspect of the psoas of the transverse processes. The two parts of the muscle to the median sagital plane, distance of the posterior as- fused to form the psoas major. However, near the ver- pect of the psoas to the skin, distance of the anterior tebral bodies, these two parts were separated by a thin aspect of the psoas to the skin and the estimated distance fascia within which were the roots of the lumbar plexus of the femoral nerve to the skin. The radiological doc- and the ascending lumbar vein. The fu'st lumbar ventral uments were from 22 adults (13 women and nine men) ramus divided to give the iliohypogastric and ilioinguinal studied for various pathological conditions. All patients nerves that left the psoas muscle to travel anteriorly to had a normal lumbosacral region. the quadratus lumborum. It is noteworthy that these nerves were the only elements of the lumbar plexus in Results contact with this muscle. The genitofemoral and the lat- eral femoral cutaneous nerves originated from the second Cadaver dissection lumbar ventral ramus. The first descended on the ventral The lumbar plexus (Figure 2) originated from the first aspect of the psoas major muscle while the latter crossed four lumbar ventral rami that joined within the substance the lateral border of the psoas major at the level of the 482 CANADIAN JOURNAL OF ANAESTHESIA FIGURE 2 Lumbar plexus anatomy. (!) Quadmtus lumborum muscle, (2) lateral femoral cutaneous nerve, (3) iliolumbar ligament, (4) psoas major muscle, (5) iliacus muscle, (6) femoral nerve, (7) obturator nerve, (8) psoas major fascia. inferior margin of the fourth lumbar vertebra. The large nerve lett the psoas muscle on its internal and posterior posterior divisions of the ventral rami of I4-3-4 united side between L5 and SI. All measurements made following to form the femoral nerve. It was median to the lateral the transverse section can be found in Table I. femoral cutaneous nerve. The femoral nerve left the psoas During dissection, variations were found concerning major from its postero-lateral border at the junction of the position of the three main nerves of the lumbar plexus the muscle's upper two thirds and lower third. It then within the psoas major (Figure 3). The lateral femoral traveled in the gutter between the psoas major and iliacus cutaneous nerve and the femoral nerve were within the muscles. The anterior divisions of 14-3-4 were smaller psoas major substance at the junction of the posterior and gave rise to the obturator nerve. The obturator nerve, third and anterior two-third and in a constant relationship being median to the femoral nerve, was the inner- with one another. ,The obturator nerve localization was most nerve of the lumbar plexus. The obturator more variable. In two of the four cadavers it was in the Farny el al.: LUMBAR PLEXUS ANATOMY 483 TABLE 1 Dissection data Numbers corresponding to Figure 1 Sex Age Weight 1 2 3 4 5 6 Female 16 50 5 6 3.5 5.5 3 3 Female 32 70 6 7 4 5 3 3 Male 69 62 6 7 4.5 6 3 3.5 Male 25 80 7.5 9 5.2 7 3.5 3.8 All values (except age = years and weight = kg) are in cm. Numbers (corresponding to Figure 1): (1) distance skin/transverse process L 5, (2) distance skin/lumbar plexus nervous trunks, (3) distance nervous trunks/median sagital plane, (4) distance psoas major lateral border/median sagital plane, (5) psoas major frontal diameter, (6) psoas major median sagital diameter. FIGURE 3 Variations in the position of the lumbar plexus main nerves within the psoas major muscle at the !.~_ 5 intervertebral disc level. same fascial plane as the two other nerves, while in the 2.12 cm), distance medial aspect psoas to median sagital other two dissections it was in a fold of the psoas major plane (2.73 5:0.64 cm), distance lateral aspect psoas to different from the one enclosing the two other nerves.